1. Field of the Invention
The present invention relates to a thermal recording method for recording an image on a recording medium (e.g. , normal paper, thermal paper, processed paper, or an OHP sheet) by using heat and a recording apparatus using this thermal recording method and, more particularly, a thermal recording method capable of recording a halftone image on the recording medium and a recording apparatus using this thermal recording method.
The recording apparatuses include an electronic typewriter, a copying machine, a facsimile machine, and a printer. The recording methods include (A) the so-called ink-jet method, i.e., a method of applying heat to change a state of a recording liquid, and injecting the recording liquid from an injection port on the basis of the change in state to form an ejection droplet, thereby recording an image on a recording medium, (B) the so-called thermal printing method, i.e., a method of causing color development of thermal paper to perform recording, and (C) the so-called thermal transfer method, i.e., a method of transferring an ink of an ink ribbon to a recording medium by heating, thereby performing recording.
2. Related Background Art
In a multi-harmonization or a multi-gradation thermal transfer recording apparatus capable of recording a halftone image by a multi-gradation thermal transfer recording method (e.g., a printer apparatus for performing printing with a thermal head), a printing density is controlled by energy supplied to the thermal head, i.e., by a head voltage level or its energization time.
In a conventional multi-gradation thermal transfer recording method, however, when N-level printing is to be performed, data corresponding to the number of heat generating members of the head, e.g., 512 data, must be transferred or transported N times. For example, if image data is 6-bit data, this data is transferred 64 times. If image data is 8-bit data, this data must be transferred 256 times, which is four times the transfer cycle count of the 6-bit data. For this reason, the printing time has minimum limitations, and it is therefore difficult to shorten the printing time beyond this minimum.
When a highly precise image is recorded, the number of bits of image data is increased. The number of parallel/serial converters for converting image data into head drive data is increased in addition to an increase in recording time, thus complicating a circuit arrangement for generating signals input to the thermal head.
In order to shorten the recording time, a method disclosed in Japanese Patent Laid-Open (Kokai) No. 57-57682 (published on Apr. 6, 1982 in Japan) is known to those skilled in the art. According to the basic principle of this method, image gradation data is transferred bit by bit. The number of data transfer cycles can be reduced, and the data transfer time can be shortened accordingly. However, energization of heat generating members is not continuous. Even if the heat generating members are energized within an energization time corresponding to the number of gradation levels, data having a high gradation level may result in a lower printing density than data having a low gradation level because of heat dissipation of heat generating members in the time interval during which they are not energized.